Tray designs for vertical oriented polymerization reactors usually employ gravity and vertical drop to achieve the desired degree of polymerization without the complexities of mechanical agitation. In such tray designs, the polymeric melt cascades down the inside of the vertical length of the reaction vessel. Baffles or trays are mounted in the vessel to provide retention of the polymer melts, thereby increasing liquid residence time within the reactor and its exposure to the reaction conditions. The liquid residence time is required to allow sufficient time for the polymerization kinetics to keep up with the enhanced byproduct liberation rates achieved by the increase in the liquid-vapor surface area and the enhancement of its renewal.
U.S. Pat. Nos. 4,196,168, 3,841,836, 3,509,203, 3,359,074 and 3,787,479, and Great Britain Pat. No. 1320769 disclose reactors wherein the reacting media flows by hydraulic gradient. U.S. Pat. No. 4,196,168 discloses a vertical polymerization reactor having a series of downwardly sloping rectangular trays for conducting a flow of liquid polymer in a descending path. U.S. Pat. No. 3,841,836 discloses a vertical polycondensation reactor having a series of downwardly sloping adjustable rectangular trays and a means for continuously sensing the viscosity of the polymer. Disadvantages associated with the use of rectangular trays are that polymer distribution uniformity across the width of rectangular trays is difficult to achieve with large trays and that 30% of the reactor vessel's cross-sectional area, as compared to a circular tray, is lost. In contrast, simple cross-flow on a circular tray fit into the vessel will result in large stagnant flow regions on the periphery of the tray outside the direct streamlines from the inlet to the outlet. Liquid polymer in stagnant flow regions tends to overcook, obtain a high viscosity, crosslink and/or degrade.
U.S. Pat. No. 3,509,203 discloses a vertical reactor having a series of horizontal structures in cascade arrangement which contain a plurality of annular passages in each tray for horizontal flow of the liquid polymer therethrough and a connecting tube from the center of each horizontal structure for downward travel of the liquid polymer. Disadvantages associated with the reactor disclosed in U.S. Pat. No. 3,509,203 are that the flow path is too long for high viscosity materials, corners in the flow path are sources of stagnant flow regions, the roof inhibits vapor transmission and adds unnecessarily to the complexity of construction, and the tubes for flow between the trays prevent the free flow of a film which would enhance the liberation of vapor.
U.S. Pat. No. 3,359,074 discloses a vertical polycondensation reactor having a series of circular trays containing substantially equally spaced, chordwise extending slots. The slots are operative to generate the necessary surface renewal within the relatively more viscous fluid medium passing therethrough. Disadvantages associated with the reactor disclosed in U.S. Pat. No. 3,359,074 are that there is no positive hold-up volume on the trays, the residence time is controlled by fluid viscosity and flow rate, and the slots must be sized accurately for a particular liquid rate and liquid physical properties or the tray could flood or completely drain out. In addition, the vapor flow control mechanism is subject to plugging.
U.S. Pat. No. 3,787,479 discloses a vertical reactor having a series of circular trays with transverse baffles creating several approximately rectangular segments. Thus, the trays contain an elongated side-to-side flow path for plug-like flow of the reaction medium from one side of the tray to the other. Disadvantages associated with the reactor disclosed in U.S. Pat. No. 3,787,479 are that corners in the flow path are sources of stagnant flow regions, and the tubes for flow between the trays prevent the formation of a free-flowing film to enhance the liberation of vapor.
Great Britain Pat. No. 1320769 discloses a reactor having substantially horizontal spiral flow channels, open along the top wherein the side walls of the spiral flow channel are constructed as closed flow ducts. Disadvantages associated with the reactor disclosed in Great Britain Pat. No. 1320769 are that the use of horizontal spiral flow channels without reverse turnarounds prevents the equalizing of flow paths and the "inside" track is shorter than the "outside" track. In addition, the use of only one tray greatly limits the available free surface area, and prevents the formation of a free-falling film between trays which enhances the liberation of vapor.
Thus, the above-mentioned references are deficient in that they include either (a) stagnant flow regions caused by material being bypassed by material flowing within a shorter-path streamline, or (b) poor utilization of the circular space within horizontal cross-section of a vertically oriented cylindrical vessel.
In contrast, the reactor of the present invention utilizes a circular tray which efficiently utilizes the cross-sectional area of a cylindrical reactor while providing uniform liquid polymer melt flow path lengths which minimize stagnant flow or dead zone regions. In addition, the reactor of the present invention is able to process high viscosity liquids and provide controlled residence time (liquid volumetric holdup) for the occurrence of chemical reactions. Moreover, the reactor of the present invention is designed to allow vapor traffic to escape each tray and travel to the reactor's vapor outlet along a path external to the path of the polymer flow.